The present invention concerns a system for the multiplexed transmission of seismic signals provided by a plurality of receivers in digital form.
French Pat. No. 69,06,077, filed Mar. 5, 1969 and granted to the Compagnie Generale de Geophysique (corresponding to U.S. patent application Ser. No. 16,704, filed Mar. 5, 1970, now U.S. Pat. No. 3,652,979, issued Mar. 28, 1972), and its first Certification of Addition No. 70,04,447, (British patent specification No. 1,295,332), describe a system for the multiplexed transmission of seismic signals. In that system, electrical signals arising from various receivers or groups of receivers are transmitted in a time multiplexed form to a central registration unit.
The system described thus includes a seismic transmission line leading to a central registration unit and starting from a pulse generator at the end of the line. Each receiver is associated with the transmission line through a multiplexing assembly including electronic gates to make communication between the receiver and the line, and a decoding element to control the opening of the gates in response to the specific reception of pulses from the generator, in such a way that the receiver signals arrive at the central registration unit in accordance with a time multiplex sequence defined by the succession of control pulses from the generator.
In French patent application No. 72,18,138, filed on May 19, 1972 in the name of the Compagnie Generale de Geophysique (corresponding to U.S. patent application Ser. No. 361,815, filed May 18, 1973, now U.S. Pat. No. 3,911,226, issued Oct. 7, 1975), there is described an "Installation for the multiplexed transmission of seismic signals" suitable for use with digital signals, (British patent application No. 23886/73). Each receiver then furnishes the information received by it in digital form to a multiplexing circuit mounted in series with the line.
The multiplexing circuit includes at least one shift register capable, on the one hand, of being filled by this digital information, and on the other hand, of being coupled operatively to the transmission line downsteam so as to apply the digital information in the form of successively coded pulses.
The generator emits first pulse signals which are recognisable with respect to the digital information; in each multiplexing circuit means for detecting these first pulse signals are coupled upstream in the transmission line, clock pulses being applied to the shift register after the detection of a first pulse signal.
In this previous Application, the means for applying the clock pulses to the shift register included preferably a source of clock pulses and means responsive to the detection of the first pulse signal to cause a predetermined number of clock pulses to be applied to the shift register from said source.